For laminated ceramic capacitors mounted with the use of soldering, wet plating such as electrolytic plating is typically applied onto external electrodes formed by baking to form, for example, Ni plating and Sn plating thereon, thereby providing improvements in mountability, more specifically, improvements in solderability to the external electrodes.
However, it is known that the plating solutions used for carrying out wet plating as described above have unfavorable influences on ceramic electronic components such as laminated ceramic capacitors to varying degrees.
The unfavorable influences mentioned above often appear as decreased mechanical strength of ceramic electronic components, such as crack generation during deflection of mounting substrates, reflow, or mounting. This will be described with reference to FIGS. 4 and 5.
FIGS. 4 and 5 show, in planar view, the appearance of a laminated ceramic capacitor 1 as an example of a laminated ceramic electronic component. The laminated ceramic capacitor 1 includes a ceramic body 2 in the shape of a cuboid. The ceramic body 2 has a plurality of stacked ceramic layers, and internal electrodes, not shown, arranged along the number of interfaces between the ceramic layers.
External electrodes 3 and 4 are formed on a pair of end surfaces of the ceramic body 2, which are opposed to each other. The external electrodes 3 and 4 are electrically connected to the internal electrodes. The external electrodes 3 and 4 have respective end edges 5 and 6 located on a pair of principal surfaces 7 and 8 of the ceramic body 2, which are opposed to each other, as well as a pair of side surfaces 9 and 10 thereof, which are opposed to each other.
On the external electrodes 3 and 4, plating films 11 and 12 are formed by wet plating as shown in FIG. 5. FIG. 4 shows a state before the formation of the plating films 11 and 12.
It has been determined that the crack generation mode differs between before plating and after plating, by a deflection test for measuring deflecting strength of this laminated ceramic capacitor 1. More specifically, before the plating, a crack 13 is likely to be caused so as to cross a central part of the ceramic body 2 as shown in FIG. 4. On the other hand, after the plating, a crack 14 that begins at sites of the ceramic body 2 with the end edges 5 and/or 6 of the external electrodes 3 and/or 4 located is likely to be generate in the ceramic body 2 as shown in FIG. 5.
From the foregoing, the plating solutions can be assumed to deteriorate the ceramic body 2, in particular, at parts with the end edges 5 and 6 of the external electrodes 3 and 4 located. This will be described with reference to FIG. 6.
FIG. 6 is a cross-sectional view illustrating an enlarged part of the laminated ceramic capacitor 1, which schematically shows a portion of the ceramic body 2 with the end edge 5 of the external electrode 3 located. The illustration of the plating films 11 and 12 is omitted in FIG. 6.
As shown in FIG. 6, the glass component contained in a conductive paste produces a glass phase 15 in the external electrode 3, when heat treatment is carried out. The glass phase 15 is distributed at more than one site in the external electrode 3. Furthermore, in the heat treatment step, the glass component in the conductive paste penetrates into grain boundaries 17 between ceramic grains 16 of the ceramic body 2 to form a reaction phase 18. Although not shown in FIG. 6, a similar phenomenon is also caused on the other side with the external electrode 4, as in the case of the side with the external electrode 3 as shown.
Then, when a plating step is carried out for forming the plating films 11 and 12 shown in FIG. 5, the glass component that penetrates into the grain boundaries 17 between the ceramic grains 16 and is located near the end edges 5 and 6 of the external electrodes 3 and 4 easily comes into contact with plating solutions, and is dissolved in the plating solutions, and as a result, the ceramic body 2 is eroded near the end edges 5 and 6 of the external electrodes 3 and 4. The generation of the crack 14 mentioned above can be assumed to be caused by brittleness due to the erosion.
On the other hand, in order to prevent erosion caused by plating solutions, it has been also proposed that the conductive paste for use in the formation of external electrodes is improved in composition. For example, Japanese Patent No. 4577461 (Patent Document 1) discloses a conductive paste containing glass frit in which SiO2 is 7 weight % or more and 63 weight % or less. However, when the conductive paste containing the glass frit with such a composition is used to try to form external electrodes, thermal diffusion of glass into the ceramic body during heat treatment for the formation of the external electrodes diffuses the ceramic component in the glass, and there is thus a possibility that properties of the glass will be changed to decrease the dissolving resistance or acid resistance against plating solutions.
It is to be noted that while the problem mentioned above is attributed to plating solutions for use in the case of forming plating films on the external electrodes, similar problems can be also created by not only the plating solutions, but also other causes. Therefore, laminated ceramic electronic components with external electrodes subjected to no plating can encounter similar problems.
Patent Document 1: Japanese Patent No. 4577461